The operation of the electrohydraulic servovalve, of the type for which the present invention is particularly useful, is set forth in detail in U.S. patent application, Ser. No. 328,058 entitled Electronic Compensator For An Electrohydraulic Servovalve, by Applegate et al., filed on Dec. 7, 1981, and assigned to the same assignee as the present invention. This Applegate disclosure is incorporated by reference herein for a thorough exposition of the operation of an electrohydraulic servovalve.
In brief, the operation of an electrohydraulic servovalve includes the employment of a drive system which functions to supply a constant drive signal I.sub.c (s) to the armature of a torque motor of the electrohydraulic servovalve. It is desired that the drive signal I.sub.c (s) developed by the drive system be proportional to an input control signal E.sub.c (s) which is inputted to the drive system. In this manner, the torque developed by the torque motor is proportional to the input control signal, and the operation of the electrohydraulic servovalve is thereby controlled.
In a typical construction of a drive system, it is implicitly assumed that the operation of an electrohydraulic servovalve is independent of the range of input frequencies of the drive signal I.sub.c (s) and therefore may be modeled as a resistive device. Based on this assumption, the drive system is designed so that its associated transfer function G(s) which is defined as ##EQU1## is a constant, and therefore independent of frequency.
In a typical employment of an electrohydraulic servovalve, the range of input frequencies of the drive signal I.sub.c (s) is relatively small. For this case, the electrohydraulic servovalve performs in a satisfactory manner, even though it is modeled as a purely resistive device and the drive system transfer function is a constant. When high input frequencies are inputted, however, to an electrohydraulic servovalve that is modeled as a resistive device, it exhibits gross instabilities and tends to be quite sensitive to component tolerances.
For a state-of-the-art employment of an electrohydraulic servovalve, for example in the field of flight simulation, it is important that an electrohydraulic servovalve be capable of satisfactorily handling a relatively broad band of input drive signal frequencies. However, electrohydraulic servovalves that are now commercially available are based on the resistive model, and therefore exhibit the indicated instabilities when high frequencies are inputted to the electrohydraulic servovalve. Consequently, commercially available electrohydraulic servovalves do not adequately fulfill state of the art requirements.
The present invention provides an electrohydraulic servovalve with associated electronic compensation apparatus that addresses the cited difficulties and fulfills the state-of-the-art requirements. In particular, the present inventor has developed an improved model for an electrohydraulic servovalve and incorporates the benefits of this improved model in novel compensation apparatus associated with the drive system, so that an electrohydraulic servovalve can satisfactorily accept a broad band of input frequencies.
The present invention is particularly suitable for employment with relatively large, two-stage electrohydraulic servovalves that are driven by input drive signals having a broad band of frequencies (e.g., 0-10,000 hz). The associated compensation apparatus ensures that the torque developed by the torque motor is proportional to the input control signal for the broad band of drive signal frequencies.